Seismic exploration system



April 3, 1951 A. J. HERMONT ETAL 2,547,703

SEISMIC EXPLORATION SYSTEM v Filed July 10, 1948 2' siaeeissheet 1 -|eLow Levei Conh-ol-B Medium Leve\ Cohi'roi L Hiqh Level Con'l-r'ol-M}-----I Def-ador- \nven+ors AW-r'ed J Mari-non? Jerome C. Toups PatentedApr. 3, 1951 SEISMIC EXPLORATION SYSTEM Alfred J. Hermont, Houston,

Toups, Bellaire, Tex.

and Jerome 0.

, assign'ors to Shell Development Company, San Francisco, Calif acorporation of Delaware Application July 10, 1948, Serial No. 38,014

3 Claims. ((11. 346-33) l This invention pertains to seismic explorationand relates more particularly to a method and a system for automaticallycontrolling the volume, gain or energy level at which a recordingseismograph operates.

In seismic recording, the relative intensities of the signals producedby seismic detectors, or rather the relative energy levels of signalinput may vary, throughout a single operational period, by factors suchas 10, 100, 1000, 10,000 or more, or, to use another method of notation,by 20, 40, 60, 80 or more decibels. It is thefunction of thetransmission and amplification network interposed between the detectorsand the recorder to apply-to these signals or signal levels a suitableamplification or attenuation control in such a manner that a legible,significant and substantially undistorted seismogram may be producedbyithe recorder.

A It is therefore an object of this invention to provide an improvedseismic recording system with an automatic volume or gain controlcircuit capable of handling input energy level variations of the orderof '70 or more decibels with a substantially flat output energy levelcharacteristic. It is also an object of this invention to provide aforward-acting control system of the above type. It is also an object ofthis invention to provide a system of the above type wherein the desiredcontrol is achieved by means of a plurality of control sections orstages arranged in cascade, said sections comprising non-linear resistoror semiconductor elements determining the range and operation thereof.

It is also an object of this invention to provide a control systemhaving a substantially constant reaction time, this term being used todefine the time required to restore the output level to approximately75% of its normal value after said normal value had been exceeded by aninput signal of any given intensity. It is also an object of thisinvention to provide an automatic seismographvolume control systemcapable of effective operation without presuppression and substantiallyfree of phase shift.

These and other objects of this invention will be understood from thefollowing description, taken' with reference to the attached drawings,wherein:

Fig. 1 is a simplified circuit diagram of the present system;

' Figs. 2 and-3 are simplified diagrams of equivalent singleandmulti-section transmission and control circuits or units.

" 4 isagraph showing comparative time-at- 2 tenuation curves for singleand multi-section control systems.

Fig. 5 is an idealized diagram of attenuation effected by amulti-section control system Referring to Fig. 1, a detector ll,suitably buried or disposed in the ground, translates seismic waves intoelectrical impulses or signals which are transmitted, through thetransmission or amplifier-controlled circuits of the present invention,to a 'galvanometer string or recording element of the recorder l2. It isunderstood that seismic exploration systems used in the field normallyemploy a plurality of seismic detectors.

The present invention is equally operative with any desired number ofdetectors, but will be described for simplicity with regard to a systemcomprising a single-detector, -'as shown in Fig. 1."

The transmission and amplification circuit having conductors l3 and 23connected between the detector 'H' and the recorder l2, comprises aplurality of control sections connected in cascade. Although any numberof such sections may be used according to this invention, it ispreferred that such number be not less than three: for example, ahigh-level control section M, a medium level section I0 and a low levelsection [8.

The high-level control section I4 comprises essentially a resistance 24and a balanced bridge 34 connected across the conductors l3v and 23 andhavinga variable non-linear resistance element in each of the four armsthereof.

Non-linear resistances are resistances whose ability to pass electriccurrent is a function of the intensity and polarity of the currentflowing 'therethrough, such for example as selenium or copper oxidesemiconductor elements. For low intensities ofcurrent passed in apredetermined (forward) direction, and for low intensities of currentpassed in the opposite (backward) direction, non-linear resistanceelements have extremely high values of resistance, while for high erintensities of current passed in the forward direction, they haverelatively low values of resistance. For example, a copper oxiderectifier may have a resistance of over 30,000 ohms for a forwardcurrent of 0.0001 milliampere, and a resistance of only about 50 ohmsfor 'a forward current of 1 milliampere. v I The resistance 24 and thebridge 34 act in con-- junction as a variable'potentiometer.

The meduim and low level control sections It and i8 comprise'similarresistances 2G and 28 and bridges 36 and38 respectively. f The high andthe medium level control sections' 14 and -"l6 are connected toeach'other 3 through one stage amplifiers I and 20. Connected a:rossleads l3 and 23 between amplifiers I9 and 29 is a signal limiter lhaving two parallel branches each comprising a non-linear resistanceelement connected to pass current in a direction opposite to that of theother branch.

The medium and low level control sections it and I8 are similarlyconnected to each other by means comprising amplifiers and- 4G and asignal limiter l1.

The low level control section [8 is connected to the recorder l2 bymeans of a high-pass filter 19.

Connected to the conductors l3 and 23 through a potentiometer 22 is acontroller circuit comprising high, medium, and low level controllersections or stages 4, 6 and 8 corresponding to and associated with thecontrol sections I l, I 6 and I8 to form control units.

The high level controller section comprises an isolation transformer 84,a rectifier i k-which may be of the vacuum tube, copper oxide or anyother desired type, a current limiter S comprising a single element of atype similar to those used in the current limiters [5 or 11, saidcurrent limiter 64 being connected across the output of rectifier M insuch a manner as to present a relatively low resistance load to therectifier and thereby to cause the control current to reach a suitablelimit at a suitable volume level, and a filter comprising a condenser 54and a choke 45.

1 The high-level controller section connects the rectifier M to thebridge 34 in such a manner that the output current of the rectifierpasses as a control current in a forward direction through the twoparallel non-linear resistance branches thereof, said branches beingconnected at their mid-points to conductors l3 and 23, as stated above.

- The medium and low-level controller sections 6 and 8 are arranged in amanner identical to that described with regard to the high levelcontroller section 4.

Connected into the controller circuit ahead of the sections 4, 6 and 8thereof are amplifiers 50, 60 and 10 respectively, amplifiers 6B and 10connecting the high. medium and low controller sections to each other.Connected across the controller circuit between sections 5 and 6 is asignal limiter 5, similar to the signal limiter l5. Another signallimiter l is similarly connected between sections 6 and 8.

Y Connected across the low level bridge 38 is an initial suppressorcircuit comprising an adjustable resistance 4|, normally open relaycontacts 42 and a may circuit to be described in greater detailhereinbelow.

The operation of the present system is as follows: 5

Upon the detonation of an explosive charge at, the shot point, thedetector II will produce, under the influence of the seismic waves fromthe shot point, a series of electric impulses or signals. As statedabove, the relative intensity of the energy levels of these signalsvaries by a factor as high as 10,000 or more, that is, by 80 decibels ormore. In order that a legible and significant seismogram may be producedby the recorder, the present system must therefore apply an automaticvolume or gain control in a rangefrom zero to about eighty decibels, alevel of zero decibels being assigned to a permissible deflection of therecording galvanometer element.

For this purpose, the signals from the detector are transmitted to therecorder I2 through the main transmission channel comprising the iiidesired amplification stages 10, 20, 30 and 40. At the same time, thesesignals are also fed to the controller channel comprising amplifiers 50,60 and 10. As seen from Fig. 1, the two channels are linked together bymeans forming the high, medium, and low level control units. The signalspassing into the controfler channel are transmitted by transformers 8 3,86 and 88 to rectifiers Hi, 16 and 18. The direct current output ofthese rectifiers is then passed as a control current through the bridges34, 36 and 38, comprising, as stated above, non-linear elements whosenormally extremely high resistance drops under the effect of saidrectified current flow to a relatively low value. The bridges 34, 36 and38 thusform relatively low resistance shunts for voltages across themain transmission channel and thus serve to attenuate excessively strongsignals.

Since the attenuating or reistance reducing effect of the attenuatorbridges is determined by the instantaneous values of the non-linearresistance elements thereof, which in turn depend on the instantaneousvalues of the control direct current passed therethrough by therectifiers, and since the intensity of said control current depends onthe magnitude of the signals supplied to the controller channel by thedetector, it will be seen that an essentially automatic volume or gaincontrol is provided by the system of. Fig. 1 for the signals transmittedto the recorder l2.

It should be particularly noted that an essential feature of the presentinvention resides .in effecting the desired signal attenuation andcontrol by means of a plurality of level control sec-, tions or stages,rather than by means of a single s age.

As stated above, the input signal level variations are often of theorder of 8.0 decibels, with an initial level sometimes reaching as highas 100 decibels. However, the range over which control must be exercisedduring normal recording does not exceed '70 decibels, the balanceconsisting not of useful energy, but of effects such as are due todirect and refracted waves, etc,

It can be shown both experimentally and analy'ically that with the typesof copper oxide or selenium non-linear re'istance elements such as areused in field work for practical reasons, an almost ideal forward-actingsteady state control characteristic can be obtained for only a rangeo'fabout 20 to 25 decibels. That is, if the level of the input signals fromthe detector H increases by 20 decibels, the direct current fromrectifier 18 may be made to increase as a function of said level, insuch a manner as to reduce the resistance of the bridge 38 by an amountalmost exactly sufilcient to result in an additional loss of 20decibels, thereby maintaining said output level substantially constant.Since it is desired to ex ercise control over a total range of, aboutdecibels, it will be seen that the desired result may be readilyachieved by using three control sections in cascade.

Another advantage in providing multiple level control sections residesin the fact that, due to technical limitations, the non-linear elementsincluded in bridges 3t, 36 or 38 are not sufliciently identical topermit each of said bridges tobe balanced with the desired accuracy.line-rectification ripple which is present in the direct currentsupplied to the control bridge, in spite-of fiter elements such as Mland 54, as well as the transient character of said control current,tends to produce an unbalance voltage across the contierbridge at the'poin'ts'whe're 'the latter is connected to the line. -Since thisunbalance voltage would necessarily be amplified by the followingamplification stages, the output current of the main transmissionchannel would contain both harmonics'a'nd transients and would thusdefeat the desired requirement of fidelity of transmission were itattempted to effect the whole desired attenuation by means of a singlecontrol unit.

The main reason for using a multiple stage control in the present systemis however as follows:

Fig. 3 shows an equivalent oversimplification of the transmissionchannel of Fig. 1. The three variable resistances X (34, 36 and. 38) maybe assumed to "vary simultaneously in accordance was the exponentiallaw:

ues vhf resistances 3,4, 36 and 38; and X0 and Xm are 1values of Xchosen so that X=X0 for i=0,

X;X0Xm for t= j For certain numerical values, the attenuation achievedis shown. as curve A in Fig. 4. If now the'ifcuit of Fig. 3 is changedto another equivalent circuit such as shown in Fig. 2, and the values ofX are chosen so as to produce the same finaf'attenuati'o'n while keepingthe same time cfonstant, then the attenuation achieved by the circui'tofFig. 2 is shown as curve B in Fig, .1-. From curvesA and B of Fig. 4itwill be seen that alcon siderablyfaster control may be achieved by eansof a-mul'iDle control unit circuit, which is; 'apable'to p'roduc'e,.forexample, an attenuation of 30'.decibels in .15 second, than by means ofa sfingl'control section circuit, which requires .375 secondfor the samepurpose. This is of con- :siderable importance because although theselection of the time constant is determined by other considerations, anincrease of the initial rate'of attenuation may be achieved according tothis invention by using a plurality of control sections in cascade orseries.

The same situation prevails in case of a sudden decrease or even totalcollapse of the input signal level: an three control sections actsimultaneously to remove the attenuation previously applied, therebygiving a considerably shortened reaction time.

""It should be noted in this connection that the finitial reaction time,that is, the period of time immediately following the reception ofinitial ener'zy during which no attempt is made to controlthedefiections of the galvanometer elements, psu'a'uy extends forapproximately .15 second During the remainder of the control period. thereaction t me should be as nearly constant as possible for all levels ofsignal input and should be governed by requirements arising only fromSeismological considerations, and not from re- -duire'ments dictated bythe necessity of bringing the .System under control, of minimizing the'rectificati on ripple, etc. This reaction time, referring for exampleto a step-wise-change of about 20 d e'cibels, should be approximatelfrom .2 to .3 second. That is, irrespective of whether the level of Sinal input to the main transmission channel of..Fig-., 1 abruptlyincreases or decreases by 20 .-decibels,.the time for the outputlevel ofsaid eha'nnel to reach its- -fi-nal controlledvalue-lo wihtin $2.5decibels or its ormal value; should ent system may be briefly summarizedas follows; As soon as the level of the signal input from the detector Hbegins to increase from the predetermined zero decibels level, the lowleve1-sec' tion I8 begins to exercise control, the medium and high"level sections I6 and M remainingas yet inoperative. This is due to thefact'that the signal input travelling through the control: channel issubjected to repeated amplification at amplifiers 50, Bil and ill beforereaching trans-' former 88 of the low level section. As soon-as the lowlevel section it begins to operate, the direct current output of therectifier 18, which is a direct function of the level ofsaid signal in-'put, is passed as a control current through the bridge 38, thus reducingthe resistance of saidbridge and thereby introducing an additionallossor attenuation in the main transmission channel. In other words, itmay be said that a sufficient electric loss is imposed on or injectedinto the main transmission channel at bridge 38 to offset the effect ofthe rise of the signal level from detector H. With a proper design ofthe control units, substantially linear control may be obtained, thatis, any increase in the level of] the input signal, as expressed indecibels, will result in an injected loss or attenuation in the maintransmission channel of a substantially equal decibel value, so that theoutput level re mains at the predetermined or zerovalue.

Since, as explained above, it is'essential that each of the controlsections exercise controlonly' within its predetermined "range ofinputsig'nal level, the low level control section 18 is, designed, forexample, to maintain control within a suit able low input level range,such as from zero to 25 decibels.

This means in effect that for a level of signal input below thatdetermined upon, as zero, the rectification of the signals by rectifier'48 will not produce a control. direct current capable of ap.

preciably affecting the resistance of the bridge 38, but for a signalinput level between zero and 25 decibels, the low level sectionwillexercise complete control to give an attenuation numerically equal,in decibels, to the rise of the input level, thereby maintaining theoutput level at Zero.

When the input signal level, however, reaches some value in excess of 25decibels, for example, 30 decibels, the low level section becomes unableto produce an attenuation of corresponding magnitude. This is due to thefact that the intensity of the signals reaching amplifier 'iil is outoff at a predeterminedvalue by the signal limiter 7, .and theintensityofthe'control direct current supplied from said rectified signals to thebridge 38 is further out off at a. predetermined value by the currentlimiter G8. The intensity of the control current passing through thebridge 38 can therefore never exceed a predetermined maxi mum valuenecessary to give a maximumpo'ssible attenuation of 25 decibels,which'is in agree-1- ment with the physical characteristics andpossibilities of the non-linear resistor elements, as explainedhereinabove.

When therefore the input signal level reaches in the above-assumedexamplaa value of 30 decibels, the low section 58 continues to provide acontrol over the 0-25 decibel range, while the control of the-25 -30decibel=range passes to the medium level control section It. Namely,whenthe input signal level has a value in excess of 25 decibels, theintensity of said signals is sufliciently high to cause the rectifier'16 to produce a control direct current which is supplied to the bridge36, and causes said bridge to exercise, within the 25-50 decibel range,a control action similar to that described with regard to the 025decibels range of the low level bridge '33.

When the input signal level reaches a value in excess of 50 decibels,the high level control section I4 beings to operate in a manner similarto that of the two first sections. Control is then applied by the systemin the following manner: section lfl supplies control within the 0-25decibels range, section it within the 25-50 decibels range, and sectionit within the 5D-75 decibels ran e.

The action of the present control system may be further illustrated asfollows with regard to the idealized diagram of Fig. 5 showing therelation between the input level and the attenuation (or injected loss)introduced by the present sys" tem.

It is seen that as the input level rises from zero to 25 decibels, thelow section i8 introduces a corresponding attenuation to maintain theoutput level at zero. Control is therefore eiiected along the line 0A.Point A, corresponding to an input level of 25 decibels, represents themaximum attenuation which the low level section is capable of providing.The line 0A flattens therefore at point A, as shown by line OAD,indicating that at any point between 25 and, for example, 80 decibelssignal input level, the attenuation provided by the low level section isnever more than 25 decibels.

-At the point A however, which, like point A corresponds to a signalinput level of 25 decibels, the medium section 16 comes into operationto exercise control along the line NBC. The same performance is againrepeated at 50 decibels signal input level bythe high level controlsection I4 along line B CD. This will result in a total attenuationefiected by the whole system along the line OEF, and will cause thesignal output level, as delivered to recorder i2, to remainsubstantially constant.

The advantages of this mode of operation are considerable. Assume theinput level as fiuctuating between 50 and 70 decibels. In this case,both the low and the medium level control section are statically active,that is, each provides its unvarying maximum attenuation. The high levelsection [8 is dynamically active, providing the varying amount, in thiscase about 20 decibels, of attenuation necessary over the total of the50 decibelsprovided by the two lower sections.

In the same manner, if the in ut level varies between 25 and 50decibels, only the medium level section will be dynamically active, thelow section being statically active, and the high section beingaltogether inactive.

' "It will therefore be seen that at any time and for any level ofsignal input, only one section is dynamically active. This insures thatthe system will operate with a constant reaction time, said reactiontime being simply that of any one of the single sections.

'No physical limiters actually exist Which can give a. sharp bending ofa response curve, such as the-idealized O'AB curve of Fig. 5. However, agood approximation thereto may be obtained by comiecting amplitudelimiters into the controller h nn e am le. ecnne e. tw simil rnon-linear semi-conductor elements, such pref erably as selenium orthallium copper elemer'its as shown at 5 and I in Fig. 1. As the voltageacross leads 23a and l3a increases, more and more current will fiowthrough the symmetricaltermined -or pro-set value, thereby preventingthe medium and the low control sections from operating in a range abovethat which has been assigned to them.

Since the signal at the limiter 5 (and especially at limiter l),approximates, particularly at high levels, a square wave, which is richin harmonies, and since these harmonics may find their way throughtransformers B4, and 88 to the output, appearing therein as undesirablehigh frequency distortions of a magnitude increasing with frequency, itmay be found desirable to,surround the windings of said transformerswith floating copper screens (Faraday cage) to effect an electrostaticshielding of one winding from the other.

Amplitude limiters i5 and i1, although similar to the limiters t and land used at corresponding points of the main transmission channel, havea purpose somewhat different from that of limiters 5 and E. As statedabove, no presuppression is used with the present system. Since aninitial burst of energy may have some extremely high va1ue, such asdecibels or more, and since a definite time delay, such as .15 second isrequired to bring the amplifiers under control, the grids of theamplifiers 2G, 33 and 50 may readily become positive and thus causeoverloading with the.

result of a large low frequency swing at the out ut. To prevent this,the limiters I5 and I! act as overload protecting devices duringtheinitial period, and are entirely inactive during the main, period ofthe present control circuit operation.

The reotifiers it, 76 and 18 may be oi. any desired type, such assemi-conductors of the same characteristics as in bridges 3d, 36 or 38,a full-wave rectification arrangement being preferred. The reactiveportions of each section, comprising, for example, condenser 55 andchoke G4, determine the reaction time, such as from .2

to .3 second, and also serve as rectificationripple suppressors. Insteadof a presuppressor used at the input, a suppressor arrangement may bead: vantageously used at the output of the main transmission channel toact as an adjustable sup; pressor or attenuator for the recorder i2.Such suppressor, as shown in Fig. 1, may be connected to the output ofthe amplifier l0 and comprises an amplifier 89, thyratron tubes 12 and82, and a time delay circuit comprising a condenser-'75 and variableresistances i3 and 11. As the output voltage of amplifier 10 arises uponthe arrival of a strong initial impulse, the first thyratron' F2 beginsto fire. The second thyra'f tron likewise fires after a time delaypredeter mined by the constants and the setting of alements i3, i5 andW. This energizes the relay 92 and opens the initially closed contacts42, thus connecting the variable resistor ll across the 75 maintransmission channel toj-introduce ..addi-;-

' of the record at the moment tional loss into said channel during thearrival of the first excessively strong impulses. The removal of theshunting resistance 41, which has a value dictated by designconsiderations, permits the recorder to operate at a proper sensitivityafter the passage of the initial strong impulses.

The advantage of the arrangement of resistance 41 in shunt with thebridge 38 may be illustrated by the following example:

Before the arrival of the signals from the detector, the bridge 38 mayhave a value such as 8000 ohms, and resistor 28 a, value such as 40,000ohms. Therefore, to give an attenuation of 40 decibels, resistance 4!must have a value of 67 ohms. However, after the control current fromrectifier 18 has had time to become established, the resistance of thebridge 38 will drop to a value of approximately 430 ohms, so that theparallel combination of resistances 38 and M is 58 ohms. Now, whencontacts 42 are opened, the resistance is increased to the value of thatof bridge 33 only, that is, 430 ohms, which corresponds to anattenuation of 17.5 decibels only. Therefore, the operation is asfollows: before the shot, the value of the attenuation introduced is 40decibels; after the shot it drops to 17.5 decibels, whereby 22.5decibels are regained. Thus, the contrast in attenuation isautomatically reduced as time progrosses, and an abrupt change in theappearance when the =,c ontacts 2 open is prevented.

It is understood that since the semi-conductors have a pronouncednegative temperature-resistance coefiicient of such magnitude as toaffect their performance at extreme atmospheric temperature variationsencountered in geophysical work, these units should be preferably placedin heat-insulated, thermostat-controlled container unit or unitsmaintained at a constant temperature of about 40 C., which provides thesimplest and most reliable solution to this problem.

We claim as our invention:

1. In a seismograph recording system comprising a detector and arecorder, a transmission channel between said detector and saidrecorder, said transmission channel comprising a plurality of controlsections, amplifier means connecting said control sections in cascadewith each other for amplifying the signals travelling therethrough,attenuator means in each section for attenuating the intensity of thesignals travelling therethrough, said attenuator means in each sectionbeing operative between predetermined minimum and maximum levels of thesignal input from the detector, said attenuator means comprising abalanced bridge of non-linear resistor elements shunting thetransmission channel, and means for supplying to said bridge attenuatormeans a control current as a function of the level of the signal .input,'said means comprising a controller channel connected to the detector inparallel with the transmission channel, said controller channel forminga number of controller sections correl sponding to the control sectionsof the transmission channel, means connecting said controller sectionsin cascade with each other for amplifying the signals applied thereto,and means electromagnetically linking each corresponding pair of controland controller sections to form a control unit, said means comprising-arectifier having its input connected to said controller section and itsoutput connected to said bridge attenuator for supplying thereto acontrol current, and current limiter means comprising a. non-linearresistor element for maintaining said control current below apredetermined maximum value.

2. The system of claim 1, having a plurality of control sections andcorresponding controller sections, each corresponding pair of sectionsoperating as a control coinciding with the minimum signal input level ofthe next higher unit, the range between minimum level of the lowest unitand the maximum level of the highest unit being substantially equal tothe full range of detector signal variations.

3. In a seismograph recording system comprising a detector and arecorder, automatic volume control means for said system comprising atransmission channel connected between said detector and said recorder,a controller channel connected in parallel with said transmissionchannel, means coupling mission and controller channels to form a high,medium and low level control unit, each of said control units comprisinga rectifier having its input connected to the controller channel,attenuator means comprising a balanced bridge formed cf non-linearresistor elements connected across the transmission channel, a filtercircuit connecting the output of said rectifier to said bridge to passtherethrough a control current proportional to the intensity of thesignals supplied to said rectifier, whereby the intensity of the signalspassing through the transmission channel is attenuated as a function ofsaid control current by the variable shunting action of said bridge, andamplifier and current limiter means connected in said transmission andsaid controller channels between said high and medium level controlunits and said medium and low level control units, whereby theattenuation provided by each of said control units is maintained betweenpredetermined minimum and maximum levels.

ALFRED J. HERMONT. JEROME C. TOUPS.

REFERENCES CITED The following references are of record in the file ofthis patent:

